1. Field of the Invention
The present invention relates to a method for producing a red phosphorus flame retardant, especially, a highly stabilized red phosphorus flame retardant, and to a nonflammable resinous composition containing the flame retardant.
2. Description of the Prior Art
Red phosphorus has been well known as a flame retardant for synthetic resin and has been incorporated into various resins. However, commercially available red phosphorus has hardly been used as it is. In most cases, special stabilizing treatments are required. This is mainly due to the following reasons. Namely, since red phosphorus is unstable to heat, friction, shock, etc., accidents are apt to occur during the storage and handling and incorporation of it into synthetic resin. Further, red phosphorus reacts with moisture and oxygen in the air, thereby forming toxic and harmful substances. Such desirable properties of red phosphorus do not permit its safe use as a flame retardant. Further, red phosphorus does not have a good compatibility with synthetic resin. For these reasons, red phosphorus is usually coated with an inorganic or organic material. However, in recent years, as physical properties required for synthetic resins are increasingly severe, more highly stabilized flame retardants of red phosphorus have been demanded. For example, polyolefine resins have been used as covering materials for communication cables, electric cables, etc. In such uses, serious accidents have been experienced due to fires of the cables. Therefore, a countermeasure has been urgently needed against such accidents and, at the same time, higher levels of nonflammability are required in the covering resin materials.
Conventionally, for example, polyvinyl chloride and halogen-containing polyolefine have chiefly been used as nonflammable covering resin materials. However, these halogen-containing polymers have great difficulties to ensure safety and prevent accidents, since they cause problems such as evolution of large quantities of smoke and gas, which are highly toxic and corrosive, during a fire. As means for eliminating these difficulties, it has been proposed to add a smoke inhibitor, an acid trapping filler, etc. However, these additives have to be added in large amounts to fully prevent the smoking and gas evolution and, thereby, the nonflammability inherent in the foregoing polymers may be considerably impaired. Therefore, under the existing circumstances, the halogen-containing polymers can not meet, at the same time, the requirements of reduction of smoking and environmental pollution and good nonflammability.
On the other hand, as a halogen-free nonflammable composition, there has been known a polyolefine composition prepared by incorporating a high-temperature active filler, such as magnesium hydroxide or aluminum hydroxide, which absorbs combustion heat, into polyolefine type resin. However, in the composition, these inorganic fillers are needed in large quantities in order to ensure a sufficient nonflammability, thereby causing an undesirable deterioration of the properties of the used resin, especially with respect to mechanical and electrical properties, heat resistance, water resistance and weatherability.
As previously described, red phosphorus has been well known to be an effective flame retardant for synthetic resin and has been used in practical applications related to electronics, etc.
However, red phosphorus is disadvantageous, for example, in that it forms phosphine and oxidized products with the lapse of time, thereby deteriorating the used resin. Therefore, currently, red phosphorus flame retardants composed of red phosphorus particles coated with a stabilizing agent have been mainly used. However, even the thus stabilized red phosphorus has only a very insufficient stability for a long-term use under variable environmental conditions, such as for example in cables, and, thus, can not provide good utility for such an actual use.
Nevertheless, there is still now a strong demand for further improvements in the stability of red phosphorus because of the several advantageous properties of red phosphorus. For example, when red phosphorus is used, evolution of smoke and toxic gases is very slight as compared with the foregoing chlorine-containing flame retardant. Since a considerably high flameproofing effect can be obtained with addition of small amounts of red phosphorus, addition of filler which may adversely affect the mechanical properties of resin can be avoided.
Under such circumstances, the present Inventors have considered that the known surface treatments for stabilizing have limitations and made many studies on stabilization of red phosphorus flame retardants from a quite different angle. As a result, it has been found that substantially spherical red phosphorus having entirely different surface state, physical properties and shape from any known red phosphorus can be obtained by a novel process unknown in the prior production processes of red phosphorus. Further, although the novel red phosphorus itself can be sufficiently used as a flame retardant because of its very high stability, it can be further highly stabilized by surface treating, and, thereby, exhibits significantly improved water resistance, corrosion resistance and heat resistance as compared with those of the red phosphorus flame retardant obtained in the prior art.